Heat from industries and incineration could also be captured and piped to homes, hospitals, schools, and offices. Warmth may also be sucked out rivers and the sea – and from old coalmines – using heat pumps which work like fridges in reverse. A fifth of heat needed for buildings could come from so-called district heat networks, government advisers said. These are grids of pipes laid under city streets to convey warm water generated at a centralised location by low-carbon technology.